High-pressure fluid jets, including high-pressure abrasive waterjets, are used to cut a wide variety of materials in many different industries. Abrasive waterjets have proven to be especially useful in cutting difficult, thick, or aggregate materials, such as thick metal, glass, or ceramic materials. Systems for generating high-pressure abrasive waterjets are currently available, for example the Paser® ECL Plus system manufactured by Flow International Corporation. An abrasive jet cutting system of this type is shown and described in Flow's U.S. Pat. No. 5,643,058, which is incorporated herein by reference. The terms “high-pressure fluid jet” and “jet” used throughout should be understood to incorporate all types of high-pressure fluid jets, including but not limited to, high-pressure waterjets and high-pressure abrasive waterjets. In such systems, high-pressure fluid, typically water, flows through an orifice in a cutting head to form a high-pressure jet, into which abrasive particles are combined as the jet flows through a mixing tube. The high-pressure abrasive waterjet is discharged from the mixing tube and directed toward a workpiece to cut the workpiece along a designated path.
Various systems are currently available to move a high-pressure fluid jet along a designated path. Such systems are commonly referred to as three-axis and five-axis machines. Conventional three-axis machines mount the cutting head assembly in such a way that it can move along an x-y plane and perpendicular along a z-axis, namely toward and away from the workpiece. In this manner, the high-pressure fluid jet generated by the cutting head assembly is moved along the designated path in an x-y plane, and is raised and lowered relative to the workpiece, as may be desired. Conventional five-axis machines work in a similar manner but provide for movement about two additional rotary axes, typically about one horizontal axis and one vertical axis so as to achieve in combination with the other axes, degrees of tilt and swivel.
Manipulating a jet about five axes may be useful for a variety of reasons, for example, to cut a three-dimensional shape. Such manipulation may also be desired to correct for cutting characteristics of the jet or for the characteristics of the cutting result. More particularly, a cut produced by a jet, such as an abrasive waterjet, has characteristics that differ from cuts produced by more traditional machining processes. Two of the cut characteristics that may result from use of a high-pressure fluid jet are referred to as “taper” and “trailback.” FIG. 1 is an example illustration of taper. Taper is a phenomenon resulting from the width of a jet changing from its entry into a material to its exit from the material. The taper angle refers to the angle of a plane of the cut wall relative to a vertical plane. Jet taper typically results in a target piece that has different dimensions on the top surface (where the jet enters the workpiece) than on the bottom surface (where the jet exits the workpiece). FIG. 2 is an example illustration of trailback. Trailback, also referred to as drag, identifies the phenomenon that the high-pressure fluid jet exits the workpiece at a point behind the point of entry of the jet into the workpiece, relative to the direction of travel. These two cut characteristics, namely taper and trailback, may or may not be acceptable, given the desired end product. Taper and trailback varies depending upon the speed the cut is made (the speed that the jet travels in order to produce separation of part of the material from another part) and other process parameters, such as material thickness. Thus, one known way to control excessive taper and/or trailback is to slow down the cutting speed of the system. In situations where it is desirable to minimize or eliminate taper and trailback, conventional five-axis systems have been used, primarily by manual trial and error, to apply angular corrections to the jet (by adjusting the cutting head apparatus) to compensate for taper and trailback as the jet moves along the cutting path.